After dealing with wordpress for the last two years, I know how easily wordpress can recieve viruses. All it takes is for an open door in wordpress and a hacker can insert his own scripts that make your wordpress blog the deliverer of viruses. Once Google gets ahold of the fact that your website has a virus, it will list you as a virus website.
Steps to Prevent a Wordpress Virus
1. Always update Wordpress to the latest version
2. Try making all your files on your server read only
3. Install antivirus for wordpress plugins
4. Keep a fresh backup not located on your computer (viruses you recieve on your computer can infect your wordpress files)